The evolution of strains of cells or organisms resistant to currently effective therapeutic agents is an ongoing medical problem. For example, the development of cancerous cells resistant to certain anti-proliferative agents, such as chemotherapeutic drugs, has long been recognized as a problem in the oncology field. Once resistant cells develop, the therapeutic regime must be modified to introduce other, effective anti-proliferative agents. Another example of resistance is the development of strains of microbial, fungal, parasitic and viral pathogens resistant to one or more anti-infective agents. The problem of resistance to anti-infective agents is particularly problematic for antibiotic therapy. Over the past several decades, an increasing number of bacterial strains have developed resistance to one or more antibiotic agents. Accordingly, there is a need for new anti-proliferative and anti-infective agents that are effective against strains of cells or organisms that have developed resistance to currently available agents. One approach to developing improved anti-proliferative and anti-infective agents is to provide modulators (for example, inhibitors) of ribosome function.
Ribosomes are ribonucleoproteins, which are present in both prokaryotes and eukaryotes. Ribosomes are the cellular organelles responsible for protein synthesis. During gene expression, ribosomes translate the genetic information encoded in a messenger RNA into protein (Garrett et al. (2000) “The Ribosome: Structure, Function, Antibiotics and Cellular Interactions,” American Society for Microbiology, Washington, D.C.).
Ribosomes comprise two nonequivalent ribonucleoprotein subunits. The larger subunit (also known as the “large ribosomal subunit”) is about twice the size of the smaller subunit (also known as the “small ribosomal subunit”). The small ribosomal subunit binds messenger RNA (mRNA) and mediates the interactions between mRNA and transfer RNA (tRNA) anticodons on which the fidelity of translation depends. The large ribosomal subunit catalyzes peptide bond formation—the peptidyl-transferase reaction of protein synthesis—and includes, at least, three different tRNA binding sites known as the aminoacyl, peptidyl, and exit sites. The aminoacyl site or A-site accommodates the incoming aminoacyl-tRNA that is to contribute its amino acid to the growing peptide chain. The peptidyl site or P-site accommodates the peptidyl-tRNA complex, i.e., the tRNA with its amino acid that is part of the growing peptide chain. The exit or E-site accommodates the deacylated tRNA after it has donated its amino acid to the growing polypeptide chain.
The tRNA molecule typically comprises from about 75 to about 90 nucleotides, wherein certain tRNA molecules become charged with or carry a specific amino acid. The tRNA molecule usually is represented by a two dimensional “clover leaf” (see, FIG. 1). The tRNA clover leaf comprises: an aminoacyl acceptor stem which includes a highly conserved 3′ terminal adenine nucleotide; a D stem-loop, which is so named because it generally comprises one or more dehydrouridine nucleotides; an anticodon stem-loop, which comprises a variable 3-nucleotide anticodon which base pairs (via hydrogen bonds) to a complementary mRNA codon; and a “T” stem-loop, which is so named because it often contains a ribothymidine. The aminoacyl acceptor stem includes a 3′ terminal adenosine, which by convention is designated A76, and which covalently binds to the amino acid it carries via an acyl linkage.
Because the E-site of the large ribosomal subunit plays a key role in protein biosynthesis, the E-site, like the A- and P-sites, provides an important target site for the design of modulators of protein biosynthesis, such as new anti-infective, anti-proliferative, and anti-inflammatory agents.